We have communicated on this board before and I now have information about my ancestors that seems to mesh with that of yours.
I now know that my great great grandfather Neil McTaggart who moved with his family from Bute to Illinois in 1854 was born in Campbelltown in 1804, the son of Duncan McTaggart and Charlotte McNeil. Other children born to Duncan and Charlotte were Donald (14 July 1799--known as Daniel), Christian (female, 25 January 1801), Neil (2 January 1803--apparently died as a child), Neil (4 October 1804), Catherine (20 November 1806), and Flora (15 November 1808).
Duncan McTaggart was born "about 1776 in Dalclachan, Campbeltown, Argyll, Scotland" and Charlotte McNeill "about 1777 in Campbeltown."
There were obviously many McTaggarts in Campbeltown at that time with similar names--probably close relatives.
Since the first son of Duncan and Charlotte was named Donald, the father of Duncan was likely to have had this name and to have been born about 1750. But Alexander was also a family name--given to the first son of Neil McTaggart and Margaret Montgomery.